Reinforced composite materials are widely used as structural materials for aerospace, automotive, and construction applications. These materials can provide desirable properties, such as high stiffness and strength. Composites typically include a continuous matrix phase, usually a polymeric material or a ceramic, and a reinforcement phase. The reinforcement phase can be made of inorganic materials, including metals, ceramics, and glasses; or organic materials, including organic polymers and carbon fibers. Particularly good properties are obtained when the reinforcement phase contains fibrous materials.
The manufacture of fiber-reinforced composites involves the combination of the fiber reinforcement and a liquid precursor to the matrix in a mold, followed by solidification of the liquid and formation of the matrix. This solidification can be the result of chemical reactions, in which case the liquid precursor is referred to as a reactive liquid. Alternatively, the solidification can be a physical process, for example the cooling of a thermoplastic polymer below its melting temperature.
Although the reinforcing fibers may be present in the liquid precursor prior to dispensing, better properties are typically obtained when the fibers are initially present in the mold as a preform. The liquid is then dispensed into the mold such that the final matrix fills the mold and surrounds the fibers. Preforms may be arranged as mats or meshes, and the fibers within the preform may be randomly oriented or may be oriented in one or more directions. The performance of composites is influenced by many factors, including the amount of reinforcement present relative to the matrix, referred to as fiber loading, and the degree of contact between the fibers and the matrix. Both strength and stiffness tend to be improved by an increase in fiber loading and by increased contact between the phases.
To ensure sufficient contact between the fibers and the matrix, it is desirable to use a liquid precursor which has a low viscosity. Reactive liquids are usually preferred over thermoplastics due to the low viscosity of liquids relative to polymer melts. The reactive liquid is typically a multi-component mixture; for example, the reactive liquid may contain a monomer and an activator which will cause the monomer to polymerize into a solid polymer matrix. The reactive liquid may contain more than one type of monomer, such that a reaction between the monomers produces the solid polymer matrix. Reactions between different monomers may also be facilitated by an activator.
Multi-component reactive liquids typically require complex equipment and procedures for storing and mixing the ingredients and for metering and dispensing the liquid into the mold. The ingredient ratios, mixing times, and holding temperatures are precisely controlled. The components must be thoroughly mixed so that complete reaction occurs throughout the final solid composite. Also, the mixing as well as the dispensing should be sufficiently rapid to prevent the reactive liquid from solidifying before it has filled the mold. Once the reactive liquid is ready for molding, it cannot be stored unless special precautions are taken to inhibit reactions between the ingredients. These precautions include the addition of reaction inhibitors and maintaining the liquid below a critical temperature.
For example, fiber-reinforced epoxy systems and polyurethane systems involve the use of more than one reactive monomer, and require mixing immediately before dispensing into the mold, as described in U.S. Pat. No. 4,804,427. Alternatively, fiber-reinforced vinyl ester and polyester systems involve the addition of an activator to the liquid monomer (U.S. Pat. No. 4,758,400). Fiber-reinforced poly(cycloolefin) systems typically employ two-component activators. One component of the activator is present in the monomer mixture, and the other component is added to this mixture immediately before the liquid is dispensed into the mold.
In reaction injection molding (RIM) processes, two or more reactive components are mixed together, starting the reaction between the components before the mixture is dispensed into the mold. This tends to increase the viscosity of the liquid that is dispensed due to an increase in molecular weight of the polymers or pre-polymers formed in the initial reaction. An increased viscosity can prohibit complete filling of the mold and permeation of the preform, and this tends to decrease the adhesion between the matrix and the fibers. Poor interfacial adhesion between the reinforcement and matrix phase can cause a material to have less than desirable stiffness and strength.
A variety of methods have been developed to alleviate this problem when using multi-component reactive liquids, but these methods generally increase the cost and complexity of the process. For example, the mixture of monomer and activator may be maintained below a critical temperature for reaction during the mixing and dispensing, and then the temperature of the mold can be raised above the critical temperature. The viscosity can be lowered by heating the liquid or by diluting it with a solvent. The speed of the dispensing stage can be increased by raising the injection pressure and/or by applying vacuum to the mold. In some cases, extra ingredients are employed to moderate the reaction so that complete reaction cannot occur until the material has filled the mold. These modifications to the process can provide for an increase in fiber loadings from 20-40% by volume (vol %) to 50-60 vol %.
There is thus a need for a process for making reinforced composites in which the reactive liquid feedstock does not need to be mixed, formulated, and/or metered prior to dispensing into the mold. The resulting composites will ideally contain high loadings of reinforcing materials such as fibers, have good adhesion between the fibers and the matrix, and exhibit structural properties similar to or better than the properties of composites made from conventional multicomponent systems.
In a first aspect, the present invention is a method for making a fiber-reinforced composite, comprising: dispensing a reactive liquid into a mold. The mold comprises fibers and a single-component activator on the fibers.
In a second aspect, the present invention is a method for making a fiber-reinforced composite, comprising: dispensing a reactive liquid into a mold. The reactive liquid comprises a cyclic olefin, and the mold comprises fibers and a single-component ROMP activator on the fibers.
In a third aspect, the present invention is a preform for a fiber-reinforced composite, comprising: fibers and a single-component ROMP activator on the fibers.
In a fourth aspect, the present invention is a method of making a preform, comprising: contacting a plurality of fibers with a mixture comprising a single-component ROMP activator.
In a fifth aspect, the present invention is a fiber-reinforced composite prepared by any of the above methods.
In a sixth aspect, the present invention is a fiber-reinforced composite comprising: a poly(cycloolefin) matrix, a metal, and fibers. The metal is ruthenium or osmium, and the fibers are present in an amount of 40 vol % to 80 vol %.